5193954
Description
Flashcards by Vicky Cartwright, updated more than 1 year ago
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Created by Vicky Cartwright
almost 8 years ago
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| Question | Answer |
| Why do we need TEMs? What is the resolving power? | TEM was originally developed as visible light microscopes had really bad resolutions. Resolving Power: The smallest distance between two things we can see. Our eyes have a resolution between 0.1-0.2mm. If two objects where closer together than this - they would appear as the same object. |
| How can we 'see' using electrons? | The computer converts the electron intensities into light intensities. This way we get contrast. |
| Resolution equation | |
| Approximated resolution equation | This means that the resolving power is about half the wavelength of the radiation used. E.g if green light was used (wavelength is about 550nm, then the resolution would be about 300nm) |
| De Broglie Equation | |
| What is the difference between: V and eV | V = the accelerating voltage of the microscope eV = the energy of the electrons |
| Why is it hard to build the 'perfect' TEM that approaches the resolution of the wavelength of electrons? | until recently we couldn't make very good electron lenses (like looking through the bottom of a coke bottle) |
| What does HRTEM stand for? | High Resolution TEM - we can now look at columns of atoms. - Acceleration potential is now 1-3MV - These microscope were useful for introducing controlled amont of radiation damage to samples. This was used to try and simulate nuclear reaction environments. |
| What does IVEM stand for? | Intermediate Voltage Electron Microscopes. - Operating voltage: 200-400KV - Offer a very high resolution (close to the resolution of EM with a voltage of 1MV) - Most IVEM are now HRTEM with atomic resolution. |
| What does Cs and Cc stand for? | - Cs: spherical aberrations corrected - Cc: chromatic aberrations corrected |
| What is ionising radiation? | Radiation that can remove inner-shell (tightly bound) electrons from the attractive field of the nucleus. This is done by transferring some of the energy of the incoming electrons to the individual atoms in the specimen. |
| What are the advantages of using ionising radiation? | Produces a wide range of secondary signals from the specimen which can be used in analytical electron microscopy (AEM) e.g. secondary electrons, backscattered electrons (BSE), auger electrons....... |
| What does XEDS & EELS stand for? | XEDS = x-ray energy-dispersive spectroscopy EELS = electron energy loss spectroscopy |
| What is another name for an electron beam? | An electron probe (smallest probe diameter: 0.1nm) |
| What does STEM stand for? | Scanning Transmission Electron Microscope |
| Platforms of STEM | 1.) AEM - analytical electron microscope 2.) Can be its own microscope (scanning imaging and scanned probe) 3.) DSTEM - dedicated STEM. It can only operate in the scanning mode |
| What are the uses of Cs | Smaller electron probes can be made with higher currents. - This improves analytical spatial resolution and sensitivity |
| What is the depth of field? | The amount of the object which you are imaging which is in focus. To increase the depth of field, decrease the aperture. |
| What is optical sectioning? | This is where you get slices of the image which are in focus at different depths of field. |
| What happens to the depth of field as you increase the magnification? | The depth of field decreases |
| How is depth of field useful in SEM | It allows you to see the topography of the sample. |
| Is depth of field important in TEM? | No - this is because the sample is usually all in focus at the same time as it's transparent to electrons. |
| What are the consequences of using Cs? | You can use bigger apertures, whilst keeping the same resolution, but loose the depth of field. The specimen must be super thin so that it will stay in focus in extreme conditions. |
| What happens to electron waves when they are passed through a crystal? | They are diffracted |
| Why is diffraction useful in TEM? | It tells you about the crystal structure and defects. The diffraction patterns can always be related back to the image of the specimen area. |
| How is a convergent pattern produced? | This is when the parallel beam from the TEM is converged to produce a focal probe. |
| What information can you find from a convergent beam pattern? | 1.) Complete crystal symmetry analysis 2.) Can determine point groups and space groups |
| Why can't you produce a diffraction pattern from a light microscope? | The wavelength of light is too big. Things only diffract is the wavelength is similar to the gaps. |
| Benefits of Cs in DP? Benefits of Cc in DP? | Cs: Produces sharper diffraction patterns Cc: Diffraction patterns come from smaller regions of the crystal |
| Advantages of TEM | 1.) Produces atomic level resolution images 2.) Generates a variety of signals, which give you information about the chemistry and crystallography 3.) Always produces images which are in focus |
| Limitations of TEM | 1.) Not a good sampling tool - can only look at small part of your specimen at any time 2.) Before you use the TEM you must use techniques that have better sampling e.g. light microscope and eyes |
| Interpreting TEM images | A single TEM image has no depth sensitivity. Therefore you can't tell how deep things appear in the image. There is usually info about the top and bottom surfaces, but not much about the middle. This means that you need field ion microscopy / auger spectroscopy to fully characterise the sample. |
| How to overcome the lack of depth in TEM images? | Take a load of pictures at different 3D tilts - similar to a CAT scan. This technique is used for porous materials containing catalyst particles. |
| What is the equation for intensity? | |
| When in Cc correction really useful? | Cc with energy filtering is really useful for imagining thicker samples in the TEM |
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